DC-DC converter

ABSTRACT

A Single Ended Primary Inductance Converter (SEPIC) fed BUCK converter includes: a first switch configured to open or close according to a first signal; a SEPIC portion coupled to the first switch and coupled to an energy source, the SEPIC portion comprising a first set of one or more passive components; a BUCK converter portion coupled to the first switch, the BUCK converter portion comprising a second set of one or more passive components. While the first switch is closed, the SEPIC portion is configured to store energy from an energy source in at least some of the first set of passive components and deliver energy to the BUCK portion, and the BUCK converter portion is configured to deliver energy to a load and to store energy in at least some of the second set of passive components. While the first switch is open, the SEPIC portion is configured to deliver at least some of its stored energy to the load, and the BUCK converter portion is configured to deliver at least some of its stored energy to the load.

CROSS REFERENCE TO OTHER APPLICATIONS

This application claims priority to U.S. Provisional Patent Application No. 60/992,194 entitled METHOD AND APPARATUS FOR POWER CONVERSION filed Dec. 4, 2007 which is incorporated herein by reference for all purposes; and claims priority to U.S. Provisional Patent Application No. 61/013,187 entitled METHOD AND APPARATUS FOR POWER CONVERSION filed Dec. 12, 2007 which is incorporated herein by reference for all purposes.

BACKGROUND OF THE INVENTION

Modern electronic devices often require power conversion. For example, battery operated devices such as notebook computers and mobile phones often include microprocessors that require the batteries to supply low voltages and high currents. BUCK converter is a type of step-down converter often used in DC-DC power conversion applications. FIG. 1 is a schematic diagram illustrating a conventional BUCK converter. BUCK converter 100 is sometimes referred to as a synchronized BUCK converter because switches S_(1B) and S_(2B) are synchronized to alternately turn on or off.

Conversion efficiency and transient response are important parameters of step-down converters. Conversion efficiency determines how much power is lost during power conversion; transient response determines how quickly the converter can respond to load current or source voltage changes. In the conventional topology shown in FIG. 1, it is often difficult to both increase conversion efficiency and improve transient response since switch and parasitic losses are directly proportional to the switch mode frequency, while the value of the integrating inductor L_(BUCK) determines the first order transient response and is inversely proportional.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention are disclosed in the following detailed description and the accompanying drawings.

FIG. 1 is a schematic diagram illustrating a conventional BUCK converter.

FIG. 2A is a schematic diagram illustrating an embodiment of a SEPIC FED BUCK converter.

FIG. 2B is a diagram illustrating the magnetic structure of device 200 of FIG. 2A, with attendant voltage, current, and SEPIC FED BUCK coupling identities.

FIG. 2C is a set of graphs illustrating the timing, voltage, and current identities of device 200 of FIG. 2A, with attendant timing, voltage, and current summation expressions.

FIG. 2D is a schematic diagram illustrating an embodiment of an SFB converter that is configured to perform a Gate Charge Extraction (GCE) process when the S_(1SB) switch is turned off.

FIG. 2E is a schematic diagram illustrating an embodiment of a commutation matrix included in SFB converter 200 of FIG. 2A.

FIG. 3 is a graph illustrating the turn-on or turn-off loss ratios (K) associated with a SIB switch of a conventional BUCK converter and a S_(1SB) switch of a comparatively identical SFB converter embodiment.

FIG. 4A is a graph illustrating the first order approximation of turn-on and turn-off losses associated with switch SIB of BUCK converter 100.

FIG. 4B is a graph illustrating the first order approximation of turn-on and turn-off losses associated with switch S_(1SB) of SFB converter 250, as well as attendant switch voltage, switch current, and switch power loss identities and expressions in terms of the duty cycle of the switch (D).

FIG. 5A is a schematic diagram illustrating an embodiment of a single magnetic, magnetically coupled SEPIC FED BUCK converter with attendant voltage, current, and transfer function (M) identities.

FIG. 5B illustrates the magnetic structure of converter 500 of FIG. 5A, with attendant voltage, current, and SEPIC FED BUCK coupling identities.

FIG. 5C is a set of graphs illustrating the timing, voltage, and current identities of device 500 of FIG. 5A, with attendant timing, voltage, and current summation expressions.

FIG. 5D is a schematic diagram illustrating an SFB converter during a GCE process.

FIG. 5E is a schematic diagram illustrating a commutation matrix included in SFB converter 500 of FIG. 5A.

FIG. 6A is a schematic diagram illustrating an embodiment of a multi-phase magnetically coupled, single magnetic SFB converter with attendant voltage, current, and transfer function (M) identities.

FIG. 6B is a diagram illustrating the magnetic structure of converter 600 of FIG. 6A, with attendant voltage, current, and SEPIC FED BUCK coupling identities.

FIG. 6C is a set of graphs illustrating the timing, voltage, and current identities of device 600 of FIG. 6A, with attendant timing, voltage, and current summation expressions.

FIG. 7A is a schematic diagram illustrating another embodiment of a SFB converter.

FIG. 7B is a diagram illustrating the magnetic structure of SFB converter 700 of FIG. 7A, with attendant voltage, current, and SEPIC FED BUCK coupling identities.

FIG. 7C is a set of graphs illustrating the timing, voltage, and current identities of SFB converter 700 of FIG. 7A, with attendant timing, voltage, and current summation expressions.

FIGS. 8A-8D illustrate the inductive windings in a conventional BUCK converter and in several SFB converters, with attendant current identities and dimensional expressions.

FIG. 8E is a graph illustrating the conductive loss ratios of a conventional BUCK converter and SFB converters.

FIG. 9 is a graph illustrating the inductor set/reset ratios of a SFB converter (e.g., SFB 200, 500, 600 or 700) and a canonical BUCK converter.

DETAILED DESCRIPTION

The invention can be implemented in numerous ways, including as a process; an apparatus; a system; and/or a composition of matter. In this specification, these implementations, or any other form that the invention may take, may be referred to as techniques. In general, the order of the steps of disclosed processes may be altered within the scope of the invention. Unless stated otherwise, a component such as a processor or a memory described as being configured to perform a task may be implemented as a general component that is temporarily configured to perform the task at a given time or a specific component that is manufactured to perform the task. As used herein, the term ‘processor’ refers to one or more devices, circuits, and/or processing cores configured to process data, such as computer program instructions.

A detailed description of one or more embodiments of the invention is provided below along with accompanying figures that illustrate the principles of the invention. The invention is described in connection with such embodiments, but the invention is not limited to any embodiment. The scope of the invention is limited only by the claims and the invention encompasses numerous alternatives, modifications and equivalents. Numerous specific details are set forth in the following description in order to provide a thorough understanding of the invention. These details are provided for the purpose of example and the invention may be practiced according to the claims without some or all of these specific details. For the purpose of clarity, technical material that is known in the technical fields related to the invention has not been described in detail so that the invention is not unnecessarily obscured.

Embodiments of a Single Ended Primary Inductance Converter (SEPIC) fed BUCK (SFB) converter are disclosed. The converter includes a SEPIC portion that is galvanically or magnetically coupled to a BUCK converter portion. The SEPIC portion and the BUCK converter portion share a switch. While the switch is closed, the SEPIC portion is configured to store energy from an energy source and to deliver energy to the BUCK converter portion, and the BUCK converter portion is configured to deliver energy it receives from the SEPIC portion to the load and to store energy. While the switch is open, the SEPIC portion is configured to deliver at least some of its stored energy to the load, and the BUCK converter portion is configured to deliver at least some of its stored energy to the load.

FIG. 2A is a schematic diagram illustrating an embodiment of a SEPIC FED BUCK converter. An ideal circuit without parasitic effects is shown for purposes of clarity. In this example, device 200 includes a SEPIC portion coupled to a BUCK converter portion. Switch S_(1SB) is coupled to both the SEPIC portion and the BUCK converter portion. As will be described in greater detail below, the SEPIC portion and the BUCK converter portion are galvanically coupled. The SEPIC portion includes switch S_(2S) (also referred to as the SEPIC portion associated switch) and a set of passive components including coupled inductors T_(1A) and T_(1B), capacitor C₂, as well as optional input capacitor C₁. An energy source E_(in) (such as a battery) is coupled to the inductors at input nodes A and E. The negative terminal of the energy source is sometimes referred to as the ground terminal. The BUCK converter portion includes switch S_(2B) (also referred to as the BUCK converter portion associated switch) and a set of passive components. In this case the passive components include inductor T_(1C). A load R is optionally coupled between a terminal of inductor T_(1C) and the negative terminal of the input source. Switch S_(1SB) is configured to open or close according to a first switching signal. Switches S_(2B) and S_(2S) are configured to open or close according to a second switching signal. In the embodiment shown, the switching signals are provided by a controller 206, which is optionally included in the SFB converter in some embodiments. In various embodiments, the controller may be a discrete component separately coupled to the SFB converter circuitry, or an integrated component of the circuitry. The first and second switching signals are synchronized to be opposite of each other. In other words, when S_(1SB) is open, S_(2B) and S_(2S) are closed, and vice versa. For purposes of clarity in the following discussions it is assumed that in the converter circuit, the inductors have the same inductance. Different inductance values may be used in other embodiments. The transfer function of the converter (i.e., the ratio of the output voltage E_(out) to the input voltage E_(in)) is expressed as:

M=D/(2−D),

where D is the duty cycle of the switching signal associated with S_(1SB), and where D={1−[(E_(in)−E_(out))/(E_(in)+E_(out))]}.

In the embodiment shown, an input capacitor C₁, an intermediate capacitor C₂, an output capacitor C₃ are included to provide integrating functions. S_(2B) and S_(2S) are implemented using transistors that include gate terminals. An optional drive inductor T_(1D) is included in the circuit to provide common mode drive to the gate terminal of S_(2S) to turn the transistor of S_(2S) off or on, thereby opening or closing the switch. T_(1D), however, does not substantially perform power conversion function in this example. In some embodiments, drive inductor T_(1D) is replaced with a solid state driver or any other appropriate driver.

FIG. 2B is a diagram illustrating the magnetic structure of device 200 of FIG. 2A, with attendant voltage, current, and SEPIC FED BUCK coupling identities. In this example, inductive windings T_(1A), T_(1B), T_(1C), and T_(1D) share the same magnetic core.

FIG. 2C is a set of graphs illustrating the timing, voltage, and current identities of device 200 of FIG. 2A, with attendant timing, voltage, and current summation expressions. In the examples shown, T represents a period of the switching signal, t_(ON) represents the time period during which switch S_(1SB) is closed (in other words, the transistor used to implement the switch is turned on), and t_(OFF) represents the time period during which switch S_(1SB) is open (the transistor is turned off). The duty cycle of the switching signal is represented as D.

Referring to FIGS. 2A and 2C, during t_(ON), S_(1SB) is closed while S_(2B) and S_(2S) are open. DC currents I₁ and I₃ flow through inductors T_(1A) and T_(1B), respectively. Thus, energy from the source is stored in the inductors in the SEPIC portion. During this time, the SEPIC portion does not directly deliver energy to the load, but delivers energy from the source to the BUCK converter portion. At the same time, current I₆ flows through T_(1C). Energy is therefore stored in the inductor in the BUCK converter portion. According to FIG. 2C, I₂ (graph J) is the current through capacitor C₂, and I₉ (graph P) is the current through capacitor C₃. During t_(ON), C₂ and C₃ discharge (set), and current I_(out) is delivered to the load. Thus, the BUCK converter portion delivers energy to the load during t_(ON).

Again referring to FIGS. 2A and 2C, during t_(OFF), S_(1SB) is open while S_(2B) and S_(2S) are closed. Inductors T_(1A) and T_(1B) maintain DC current flow. The SEPIC portion delivers at least some of its stored energy through switch S_(2S) to the load without substantially storing energy in its inductors. Because the closed switch S_(2S) forms an electrical path between the SEPIC portion and the load and because the electrical path has DC continuity, the energy transfer process does not require transformer action. Thus, the circuit is said to be galvanically coupled. The BUCK converter portion also delivers at least some of the energy that was stored in its inductor during t_(OFF). The BUCK converter portion, however, does not substantially store energy during t_(OFF). C₂ and C₃ charge (resest) during this period. The ON/OFF cycle is then repeated.

In some embodiments, the SFB converter is configured to implement a gate extraction process to reduce turn-off power loss and improve turn-off speed. FIG. 2D is a schematic diagram illustrating an embodiment of an SFB converter that is configured to perform a Gate Charge Extraction (GCE) process when the S_(1SB) switch is turned off. In this example, device 250 is similar to device 200 shown in FIG. 2A. Switches S_(1SB), S_(2B) and S_(2S) are implemented using metal-oxide field-effect transistors (MOSFETs). Outputs of drivers DR1 and DR2 are coupled to the gates of the MOSFETs, providing switching signals that turn the transistors off and on. Driver return terminals 262 and 264 are coupled to the sources of their respective MOSFETs. During t_(OFF), the voltage applied to the gate terminal of MOSFET S_(1SB) drops to turn the device off. Inductor T_(1A), however, will maintain its current flow, thus causing a current 268 to flow from the gate to the driver, thereby extracting charges accumulated in the gate-source capacitance of the MOSFET. Current 268 is therefore referred to as the GCE current. Since inductor T_(1B) is coupled to T_(1A), a current 270 is induced in T_(1B). Current 270, referred to as the GCE induced current, flows in a loop in the opposite direction as current 268. Currents 268 and 270 combine to form a turn-off current. The GCE process allows SFB converter 250 to have fast turn off time and low turn off loss.

In some embodiments, the SFB converter includes a commutation matrix to improve the converter's turn-on characteristics by using a capacitance set/reset process to contain parasitic energy. FIG. 2E is a schematic diagram illustrating an embodiment of a commutation matrix included in SFB converter 200 of FIG. 2A. In the example shown, commutation matrix 280 includes a set of diodes and capacitors. Nodes B, C, E, and F of the commutation matrix are coupled to nodes B, C, E, and F of SFB converter 200. Voltage identities associated with the capacitors C_(com1), C_(com2), and C_(com3) are expressed as:

E _(Ccom1) =E _(Ccom2)=(E _(in) +E _(out))/2; and

E _(Ccom3)=(E _(in) −E _(out))/2.

FIG. 3 is a graph illustrating the turn-on or turn-off loss ratios (K) associated with a S_(1B) switch of a conventional BUCK converter and a S_(1SB) switch of a comparatively identical SFB converter embodiment. In this example, turn-on or turn-off loss of switch S_(1SB) of SFB converter 250 is compared with that of switch S_(1B) of conventional BUCK converter 100. In this example, converters 100 and 250 are said to be comparatively identical since they are assumed to have switches with identical characteristics, and the same E_(in) and I_(out). The switches are assumed to turn on and off at the same rate. When the switch is turned on (i.e., the switch is closed), the voltage across the switch does not drop to zero instantaneously, therefore causes turn-on loss. When the switch is turned off (i.e., the switch is open), the current through the switch also does not drop to zero instantaneously and also causes turn-off loss. The turn-on and turn-off loss of the conventional BUCK converter 100 is assumed to be 1, shown as line 300.

A first order approximation of the ratio of the turn-on loss of the SFB converter 250 to the turn-on loss of the BUCK converter 100 is expressed as:

K _(SFBon)=1/(2−D)³,

where D is the duty cycle of the switching signal. The loss curve as a function of D corresponds to curve 302 in the figure.

A first order approximation of the ratio of turn-off loss of the SFB converter to turn-off loss of the BUCK converter is expressed as:

K _(SFBoff) =a ²/[2E _(in) ²(2−D)],

where a corresponds to a device transconductance characteristic and E_(in) corresponds to an input voltage of the converter. The loss curve corresponds to curve 304.

A first order approximation of the ratio of the total (turn-on plus turn-off) loss of the SFB converter to the total loss of the BUCK converter is expressed as:

K _(SFBTotal)=0.5(K _(SFBon) +K _(SFBoff)).

The loss curve corresponds to curve 306.

FIG. 4A is a graph illustrating the first order approximation of turn-on and turn-off losses associated with switch S_(1B) of BUCK converter 100. The attendant switch voltage, switch current, and switch power identities and expressions in terms of the duty cycle of the switch (D) are also illustrated.

FIG. 4B is a graph illustrating the first order approximation of turn-on and turn-off losses associated with switch S_(1SB) of SFB converter 250, as well as attendant switch voltage, switch current, and switch power loss identities and expressions in terms of the duty cycle of the switch (D). Since the operating current I_(D) associated with switch S_(1SB) of SFB converter 250 is significantly less than the operating current I_(D) associated with switch S_(1B) of BUCK converter 100, the turn-on loss is significantly reduced. A first order approximation of turn-on power loss associated with turning on S_(1SB) is:

P _(LossSFBon)=[0.25(E _(in) +E _(out))·I _(out)(2−D)]·T _(turn-on) ·f,

wherein E_(in) corresponds to the input voltage of the converter, I_(out) corresponds to the output current of the converter, D corresponds to the duty cycle of the switching signal, T_(turn-on) corresponds to the amount of time required to turn on the switch, and f corresponds to the frequency of the switching signal.

A first order approximation of turn-off power loss associated with turning off S_(1SB) is:

P _(LossSFBoff)=0.5a·[I _(out)/(2−D)]·T _(turn-off) ·f,

wherein a corresponds to a device transconductance characteristic (which equals 2 volts in this example), I_(out) corresponds to an output current of the converter, D corresponds to the duty cycle of the switching signal, T_(turn-off) corresponds to the turn-off time of the switch, and f corresponds to the frequency of the switching signal.

Several other SEPIC FED BUCK converter topologies exist. FIG. 5A is a schematic diagram illustrating an embodiment of a single magnetic, magnetically coupled SEPIC FED BUCK converter with attendant voltage, current, and transfer function (M) identities. Converter 500 shown in this example includes a SEPIC portion and a BUCK converter portion that are magnetically coupled. The portions are said to be magnetically coupled because there is no galvanic path for transferring energy from the SEPIC portion to the load when S_(1SB) is turned off; instead, inductors T_(1C) and T_(1D) act as transformers to transfer energy stored in SEPIC windings T_(1A) and T_(1B) to the load. FIG. 5B illustrates the magnetic structure of converter 500 of FIG. 5A, with attendant voltage, current, and SEPIC FED BUCK coupling identities.

FIG. 5C is a set of graphs illustrating the timing, voltage, and current identities of device 500 of FIG. 5A, with attendant timing, voltage, and current summation expressions.

FIG. 5D is a schematic diagram illustrating an SFB converter during a GCE process. SFB converter 550 shown in this example is similar to converter 500 of FIG. 5A. SFB converter 550 is magnetically coupled. As shown in this diagram, when S_(1SB) switches off, GCE current 568 flows in the opposite direction as GCE induced current 570, and charges in the gate-source capacitance of switch S_(1SB) are quickly removed.

FIG. 5E is a schematic diagram illustrating a commutation matrix included in SFB converter 500 of FIG. 5A. Nodes B, C, and E of the commutation matrix are coupled to nodes B, C, E of SFB converter 500. Voltage identities associated with capacitors C_(com1) and C_(com2) are expressed as:

E _(Ccom1) =E _(Ccom2)=(E _(in) +E _(out))/2.

In some embodiments, the SFB is configured as a multi-phase converter. FIG. 6A is a schematic diagram illustrating an embodiment of a multi-phase magnetically coupled, single magnetic SFB converter with attendant voltage, current, and transfer function (M) identities. In this example, converter 600 includes a first SEPIC portion comprising inductors T_(1A) and T_(1B) and switch S_(1S), and a second SEPIC portion comprising inductors T_(1E) and T_(1F) and switch S_(2S). The inductors SEPIC portions are magnetically coupled. The input and output are isolated by a transformer comprising the inductive windings. The converter further includes a first BUCK converter portion comprising inductors T_(1C) and T_(1G) and switch S_(1B), and a second BUCK converter portion comprising inductors T_(1D) and T_(1H) and switch S_(2B). The inductors in the BUCK converter portions are also magnetically coupled. Switch S_(1SB) couples the first SEPIC portion to the first BUCK converter portion, and switch S_(2SB) couples the second SEPIC portion to the second BUCK converter portion. A commutation matrix similar to what was shown in FIG. 5E is included in the converter.

FIG. 6B is a diagram illustrating the magnetic structure of converter 600 of FIG. 6A, with attendant voltage, current, and SEPIC FED BUCK coupling identities.

FIG. 6C is a set of graphs illustrating the timing, voltage, and current identities of device 600 of FIG. 6A, with attendant timing, voltage, and current summation expressions. The switching signals for switches S_(1SB) and S_(2SB) have a phase offset. The switching signals for switches S_(1B) and S_(1SB) are opposite of each other, and the switching signals for switches S_(2B) and S_(2SB) are opposite. A first switching signal controlling switches S_(1SB), S_(1S) and S_(1B) have a phase offset relative to a second switching signal controlling switches S_(2SB), S_(2S) and S_(2B). The first switching signal causes switches S_(1SB), S_(1S) and S_(1B) to operate in concert such that when S_(1SB) is closed, the first SEPIC portion stores energy, and the first BUCK converter portion delivers energy to the load and stores energy; when S_(1SB) is open, the first SEPIC portion and the first BUCK converter portion both deliver energy to the load. The second switching signal causes switches S_(2SB), S_(2S) and S_(2B) to similarly affect the operations of the second SEPIC portion and the second BUCK converter portion.

Although the above example shows a 2 phase isolated SFB converter, some converter embodiments are configured to include additional SEPIC and BUCK converter portions coupled in a similar manner to produce an N-phase SFB converter.

FIG. 7A is a schematic diagram illustrating another embodiment of a SFB converter. In this example, SFB converter 700 is magnetically coupled. In various embodiments, T_(1C) and T_(1D) may be combined into a single conductor or separated as multiple conductors. FIG. 7B is a diagram illustrating the magnetic structure of SFB converter 700 of FIG. 7A, with attendant voltage, current, and SEPIC FED BUCK coupling identities. A commutation matrix similar to FIG. 5E is optionally coupled to the converter at nodes B, C, and E. FIG. 7C is a set of graphs illustrating the timing, voltage, and current identities of SFB converter 700 of FIG. 7A, with attendant timing, voltage, and current summation expressions. As shown in current I₂ (graph J), one of the SEPIC inductors T_(1B) principally conducts current during the GCE process. Thus, SFB converter 700 experiences turn-off energy loss that is even smaller than SFB converter embodiments 200 and 500.

Compared to conventional BUCK converters, SFB converters have reduced conductive loss because of the way the inductive windings are deployed in SFB converters. FIGS. 8A-8D illustrate the inductive windings in a conventional BUCK converter and in several SFB converters, with attendant current identities and dimensional expressions. In FIG. 8A, four inductive windings T_(1A), T_(1B), T_(1C), and T_(1D) of BUCK converter 100 are shown. The inductive windings share the same magnetic core. The same magnetic windings are also present in FIG. 8B, FIG. 8C, and FIG. 8D, which correspond to SFB converter 200, 500, and 700, respectively. The windings of the BUCK converter and the SFB converters are dimensionally identical since they have the same magnetic core area, window area, and number of turns. Different converter topologies, however, result in different amounts of current through individual windings. Assuming that the converters have the same output power and include windings that have the same resistance, the amounts of energy dissipated in the windings are different since the current values are different.

FIG. 8E is a graph illustrating the conductive loss ratios of a conventional BUCK converter and SFB converters. The graph compares the loss ratios of the conventional BUCK converter 100 and the SFB converters 200, 500, and 700. It is assumed that the converters have discrete components of the same values and have the same E_(in) and I_(out). The switches are assumed to turn on and off at the same rate. The conductive loss ratio (K) is expressed in terms of duty cycle (D). The conductive loss ratio of BUCK converter 100 is assumed to be 1, shown as line 800.

The conductive loss ratio of SFB 200 is shown as curve 802 and is expressed as:

K=2(1−D+D ²)/(2−D)²

The conductive loss ratios of SFB 500 and 700 are the same. The ratio as a function of D is shown as curve 804, and is expressed as:

K=(2−0.5D)/(2−D)².

The SFB converters also have faster transient response attributes. In comparison with a comparative identical conventional BUCK converter, the transient EMF (set) volt second (Et) of the integrating inductor and the MMF (reset) volt second (Et) of the integrating inductor in the SFB converter are both lower. FIG. 9 is a graph illustrating the inductor set/reset ratios of a SFB converter (e.g., SFB 200, 500, 600 or 700) and a canonical BUCK converter. The ratio K is expressed in terms of transfer function M. Assuming that the set ratio and the reset ratio of the conventional BUCK converter 100 are both 1, which is shown as line 900. The reset ratio of a SFB converter with the same passive component values, output, and switching duty cycle is shown as line 902 and is expressed as:

K _(SFBreset)=[(1+M)/2]²,

where M=D/(2−D).

The set ratio of the SFB converter is shown as line 904 and is expressed as:

K _(SFBSet)=(1+M)/2.

Although the foregoing embodiments have been described in some detail for purposes of clarity of understanding, the invention is not limited to the details provided. There are many alternative ways of implementing the invention. The disclosed embodiments are illustrative and not restrictive. 

1. A DC-DC converter comprising: a switch S_(1SB), a switch S_(2B), a switch S_(2S), a capacitor C₂, an inductor T_(1A), an inductor T_(1B), and an inductor T_(1C); wherein: a first terminal of inductor T_(1A) is coupled to a first terminal of an energy source; a first terminal of capacitor C₂, a second terminal of inductor T_(1A), and a first terminal of switch S_(1SB) are coupled; a first terminal of inductor T_(1C), a second terminal of S_(1SB), and a first terminal of S_(2B) are coupled; a second terminal of inductor T_(1C) and a first terminal of S_(2S) are coupled; a first terminal of inductor T_(1B) and a second terminal of S_(2B) are configured for coupling with a second terminal of the energy source; and a second terminal of capacitor C₂, a second terminal of inductor T_(1B), and a second terminal of S_(2S) are coupled.
 2. The DC-DC converter of claim 1, further comprising a capacitor C1 and a capacitor C3; wherein: a first terminal of capacitor C₁ is coupled to the first terminal of inductor T_(1A) and a second terminal of capacitor C1 is coupled to the first terminal of inductor T_(1B); and a first terminal of capacitor C₃ is coupled to the second terminal of inductor T_(1C) and a second terminal of capacitor C₃ is coupled to the first terminal of inductor T_(1B).
 3. The DC-DC converter of claim 1, wherein inductors T_(1A), T_(1B), and T_(1C) are inductively coupled.
 4. The DC-DC converter of claim 1, wherein the second terminal of inductor T_(1C) is coupled to a load.
 5. The DC-DC converter of claim 1, further comprising a commutation matrix coupled to the second terminal of inductor T_(1A), the first terminal of inductor T_(1C), and the first terminal of T_(1B), and the second terminal of inductor T_(1B).
 6. The DC-DC converter of claim 1, further comprising an inductor T_(1D), wherein the first terminal of inductor T_(1D) is coupled to a gate terminal of switch S_(2B) and a second terminal of inductor T_(1D) is coupled to a gate terminal of switch S_(2S).
 7. The DC-DC converter of claim 1, further comprising a controller configured to provide a signal to switch S_(1SB).
 8. A DC-DC converter comprising: a switch S_(1SB), a switch S_(2B), a switch S_(2S), a capacitor C₂, an inductor T_(1A), an inductor T_(1B), an inductor T_(1C), an inductor T_(1D), and wherein: a first terminal of inductor T_(1A) is coupled to a first terminal of an energy source; a second terminal of inductor T_(1A), the first terminal of inductor T_(1B) and a first terminal of switch S_(1SB) are coupled; the first terminal of inductor T_(1C), a first terminal of inductor T_(1D), a second terminal of switch S_(1SB), a first terminal of switch S_(2B), and a first terminal of switch S_(2S) are coupled; a second terminal of inductor T_(1C) and a second terminal of inductor T_(1D) are coupled; and a second terminal of capacitor C₂, a second terminal of switch S_(2B), and a second terminal of switch S_(2S) are coupled.
 9. The DC-DC converter of claim 8, further comprising a capacitor C₁ and a capacitor C₃; wherein: a first terminal of capacitor C₁ is coupled to the first terminal of inductor T_(1A) and a second terminal of capacitor C₁ is coupled to the second terminal of capacitor C₂; and a first terminal of capacitor C₃ is coupled the second terminal of inductor T_(1C) and a second terminal of capacitor C₃ is coupled to the second terminal of capacitor C₂.
 10. The DC-DC converter of claim 8, wherein inductors T_(1A), T_(1B), T_(1C), and T_(1D) are inductively coupled.
 11. The DC-DC converter of claim 8, wherein the second terminal of inductor T_(1C) and the second terminal of T_(1D) are coupled to a load.
 12. The DC-DC converter of claim 8, further comprising a commutation matrix coupled to the second terminal of inductor T_(1B), the first terminal of inductor T_(1C), and the second terminal of capacitor C₂.
 13. The DC-DC converter of claim 8, further comprising a controller configured to provide a signal to switch S_(1SB).
 14. A DC-DC converter comprising: an inductor T_(1A), an inductor T_(1B), an inductor T_(1C), an inductor T_(1D), an inductor T_(1E), an inductor T_(1F), an inductor T_(1G), an inductor T_(1H), a capacitor C₂, a switch S_(1SB), a switch S_(2SB), a switch S_(1S), a switch S_(2S), a switch S_(1B), and a switch S_(2B); wherein: a first terminal of inductor T_(1A), a first terminal of inductor T_(1F), and a first terminal of an energy source are coupled; a second terminal of inductor T_(1A), a first terminal of inductor T_(1B), and a first terminal of switch S_(1SB) are coupled; a second terminal of switch S_(1SB), a first terminal of switch S_(1S), and a first terminal of inductor T_(1C) are coupled; a second terminal of inductor T_(1C) and a first terminal of inductor T_(1D) are coupled; a first terminal of capacitor C₂, a second terminal of switch S_(1S), and a first terminal of S_(2S) are coupled; a second terminal of inductor T_(1D), a first terminal of S_(2SB), and a second terminal of S_(2S) are coupled; a second terminal of inductor T_(1E) and a second terminal of inductor T_(1F) are coupled; a first terminal of switch S_(1B) and a first terminal of inductor T_(1G) are coupled; a first terminal of switch S_(2B) and a first terminal of inductor T_(1H) are coupled; a second terminal of switch S_(1B) and a second terminal of switch S_(2B) are coupled to a first output terminal; a second terminal of inductor T_(1G) and a second terminal of inductor T_(1H) are coupled to a second output terminal; and inductor T_(1C), inductor T_(1D), inductor T_(1G), and inductor T_(1H) are magnetically coupled.
 15. The DC-DC converter of claim 14, further comprising a capacitor C₁ and a capacitor C₃; wherein: a first terminal of capacitor C₁ is coupled the first terminal of inductor T_(1A) and the second terminal of capacitor C₁ is coupled to the ground terminal; and the first terminal of inductor C₃ is coupled to the first output terminal and the second terminal of inductor C₃ is coupled to the second output terminal.
 16. The DC-DC converter of claim 14, wherein inductors T_(1A) and T_(1B) are inductively coupled.
 17. The DC-DC converter of claim 14, wherein the first output terminal and the second output terminals are coupled to a load.
 18. The DC-DC converter of claim 14, further comprising a commutation matrix coupled to node B, node C, and node E.
 19. The DC-DC converter of claim 14, further comprising a controller configured to provide a signal to switch S_(1SB).
 20. A DC-DC converter, comprising: a switch S_(1SB), a switch S_(2B), a capacitor C₂, an inductor T_(1A), an inductor T_(1B), and an inductor T_(1C); wherein: a first terminal of inductor T_(1A) is coupled to a first terminal of an energy source; a second terminal of inductor T_(1A), a first terminal of inductor T_(1B), and a first terminal of switch S_(1SB) are coupled; a second terminal of switch S_(1SB), a first terminal of inductor T_(1C), and a first terminal of switch S_(2B) are coupled; a second terminal of inductor T_(1C) is coupled to a first output terminal; a first terminal of capacitor C₂ and a second terminal of inductor T_(1B) are coupled; a second terminal of capacitor C₂, a second terminal of switch S_(2B) are coupled to a second output terminal.
 21. The DC-DC converter of claim 20, further comprising an inductor T_(1D), wherein a first terminal of inductor T_(1D) is coupled to the first terminal of inductor T_(1C), and a second terminal of inductor T_(1D) is coupled to the second terminal of inductor T_(1C).
 22. The DC-DC converter of claim 20, further comprising a capacitor C1 and a capacitor C3; wherein: a first terminal of capacitor C₁ is coupled to the first terminal of inductor T_(1A) and a second terminal of capacitor C1 is coupled to the second output terminal; and a first terminal of capacitor C₃ is coupled to the second terminal of inductor T_(1C) and a second terminal of capacitor C₃ is coupled to the second output terminal.
 23. The DC-DC converter of claim 20, wherein the first output terminal is coupled to a load.
 24. The DC-DC converter of claim 20, further comprising a commutation matrix coupled to the second terminal of inductor T_(1A), the first terminal of inductor T_(1C), and the second output terminal.
 25. The DC-DC converter of claim 20, further comprising a controller configured to provide a signal to switch S_(1SB). 